Spell beings are a special type of pet added in SLASH'EM. They are created by the spellbooks of Flame Sphere and Freeze Sphere. Both spells are level 1 matter spells in SLASH'EM that are deferred features in vanilla, and both will summon the appropriate type of sphere.
Differences from regular pets
The major difference between a regular pet and a spell being is that the kills of a spell being are considered your kills, much as if you cast a spell at the monster directly. Non-fatal attacks will anger peaceful monsters. Fatal attacks on a peaceful monster will carry the potential alignment and murder penalties associated with killing the monster directly. This is especially problematic, as the two normal spell beings are mindless and will therefore attack anything that isn't tame they encounter, including shop keepers and priests. Beware the ring of conflict, as spell beings will attack other pets, and if they kill them will give you the penalty.
Another minor difference is that spell beings are immune to treachery.
Spell beings are not immune to polymorph. The polymorphed monster will retain the experience and blame transference characteristic of unpolymorphed spell beings. It is extremely easy to create an army of polymorphed spell beings using a polymorph trap due to the low level of the spellbooks; simply use a magic whistle to cycle the sphere in the trap until good forms appear. This can be used to quickly gain experience levels by obtaining an army of powerful pets. The one-time use of the flame sphere and freeze sphere is characteristic of the spheres themselves, not spell beings, so the polymorphed pet will stick around significantly longer as well.